Dads will be able to take up to six months' paternity leave while their child's mother returns to work, under government plans announced today.

Fathers will have a legal right to take the place of the mother at home for the last three months of her nine-month maternity leave.

During that time, they would be eligible for statutory government pay of £123 a week. They would then be allowed to take an additional unpaid three months off, which would effectively allow couples to have a total of 12 months' parental leave between them.

The measure would allow mothers who earn more than their partners to return to work earlier, and is a victory for Harriet Harman, the Women and Equalities minister, who has championed extra parental leave and flexible working rights for parents.

Ministers intend to push through the necessary legislation by this April - prior to the general election, which is expected to be held in May. The changes will affect parents of children due on or after 3 April 2011 in "order to give employers time to adjust".

Last year, the government postponed plans to extend paid maternity leave to 12 months and allow parents to split this allowance between them, citing the economic climate and the costs and bureaucracy it would cause employers.

Harman said: "This gives families radically more choice and flexibility in how they balance work and care of children, and enables fathers to play a bigger part in bringing up their children.

Ministers estimate that between 4% and 8% of those eligible for the new leave will take it, with only 1% of small businesses expected to be affected.

The transferable leave announcement was welcomed by the trade union movement.

TUC General Secretary Brendan Barber said the proposals would help millions balance their work and family life. "As both parents work in most modern families, it's right that mums and dads should decide who looks after their baby, rather than the state deciding for them by only paying the mother for parental leave," he said.

"Business lobby groups have opposed every new family-friendly right, from flexible working to extended maternity pay. But in reality these changes have hugely benefitted millions of families and have had no damaging effect on businesses."

The Tories speculated that the pledge would be "quietly dropped" by Labour after voters go to the polls.

Shadow Minister for Women and Equality, Theresa May said: "Labour's track record implies that this is a pre-election pledge that could be quietly dropped after voters go to the polls. Labour went into the last election promising to extend maternity leave to a year, but have broken that pledge, why should anyone trust them on paternity leave now?"

May said the proposals "trailed behind" the Conservatives' own plans, which would allow parents greater flexibility by being able to take their leave simultaneously by splitting a year's leave between them.